PET/CT Instrumentation:
Review and Updates
Osama Mawlawi, Ph.D.
Department of Imaging Physics
MD Anderson Cancer Center
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Annihilation
Detector ring
511keV
Photon
LOR
electron
positron
Nucleus
511keV
Photon
E=mC2
Electron rest mass = 9.11x10-31 kg
C=3.0x108 m/s
1 joule =1 kg m2/s2
1 e = 1.602x10-19 coulomb
1 eV = 1.602x10-19 J
Thus: E =1.02 MeV
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Ax
i al
LOR Data Storage
Modified from http://www.crump.ucla.edu/software/lpp/lpphome.html
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Ax
i al
Modified from http://www.crump.ucla.edu/software/lpp/lpphome.html
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Type of recorded events
Scatter
Random
True
Total events = Trues + Randoms + Scatter
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Sample Sinograms
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Quantification: Power of PET
Measured Data
Random subtract
Normalize
Dead time
Correct Geometry
Calculate/subtract scatter
Correct Attenuation
FBP or IR reconstruction
Ideal measured data
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P1 = e−μL1
P2 = e−μL2
P1 * P2 = e−μL
Attenuation is dependant
on the path length and not
the depth of the source
of activity
Four ways to get an
attenuation map
1) Measured (MAC)
2) Calculated (CAC)
3) Segmented (SAC)
4) CT based (CTAC)
Nuclear Medicine: Diagnosis and therapy. Harbert J, Eckelman W., Neumann R.
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Coincidence Calibrations
• Well Counter Correction (WCC)
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Coincidence Calibrations
• Well Counter Correction (WCC)
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Dedicated PET Imaging
• Emission (2D mode)
• Transmission (scans are interleaved)
• 5 to 6 bed positions
• 8 min per position
• 5 EM, 3 Tx
• Total scan duration 50-60 min
15.5 cm
Tx rod sources
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Emission
Transmission
Final
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SUV = (Measured activity * Patient weight)/Injected dose
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Disadvantages of dedicated PET
imaging techniques
• Transmission
– Noise due to low gamma ray flux from rod source
– Transmission is contaminated by emission data
• Scan duration
– Time consuming (emission & transmission )
– Increased patient movement (image blurring)
• Efficiency
– Decreased patient throughput
– Difficulty in correlating images to other diagnostic
modalities accurately
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Hybrid Imaging
GE Discovery ST
Siemens/CTI Biograph
Phillips Gemini
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PET/CT: rationale
• Short duration, low noise CT-based attenuation correction
• Improved patient throughput (< 30 min total scan duration)
• Combines functional (PET) and anatomical (CT) imaging
• PET and CT components can be operated independently
• Fully quantitative, whole-body images for SUV calculation
• Improved patient comfort and convenience (single scan)
• Improved imaging accuracy for therapy planning & monitoring
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Types of Artifacts
PET/CT imaging artifacts are due to :
•
•
•
•
Contrast media
Truncation
Respiratory motion
Metal implants
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Converting CT Numbers to
Attenuation Values
•
•
For CT values < 0 ,
materials are assumed to
have an energy
dependence similar to
water
For CT values > 0,
material is assumed to
have an energy
dependence similar to a
mixture of bone and
water
The green line shows the
effect of using water
scaling for all materials
Attenuation at 511keV
•
0.200
0.150
error
Bone/Water
0.100
0.050
0.000
-1000
Water/air
-500
0
500
1000
CT number measured at 140kVp
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New PET Scanner Features
• Mixed duration FOV
• LIST Mode acquisition
• 4D PET/CT imaging
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PET data acquisition of different duration per FOV
Static
3 min
Static
3 min
Static
5 min
Static
6 min
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LIST Mode
X2
Y1
• Time ticks are fixed at 1msec intervals
• The number of events between time ticks depends
on the amount of activity in the field of view
Y3
Y4
• The more activity, the more the events between
time ticks.
• Very flexible, data can be rebinned as static,
dynamic, or gated.
• Requires large amount of memory
X1
X4
X3
X1
Y1
X2
Y2
TIME
X3
Y3
X4
1msec
X8
Y8
X9
Y4
X5
Y5
X6
Y6
Y2
TIME
X7
Y7
1msec
Y9
X10
Y10
X11
Y11
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Gated PET (Used to image repetitively moving objects: cardiac,
respiratory)
Trigger
Trigger
1
8
2
1
7
3
4
6
8
7
2
6
3
5
4
5
time
Bin 1
• Prospective fixed forward time binning
Bin 8
• Ability to reject cycles (cardiac) that don’t match
• Single 15 cm FOV Gated PET
• User defined number of bins and bin duration
• As number of bins increase, the duration and motion per bin decreases.
However images will be noisy unless acquired for longer durations.
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New CT Application… Advantage
4D CT
Respiratory tracking with Varian RPM optical monitor
CT images acquired over complete respiratory cycle
“Image acquired”
signal to RPM
system
X-ray on
First couch position
Second couch position
Third couch position
Respiratory motion defined retrospective gating
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New PET Scanner Designs
SIEMENS (Longer AFOV, PSF, PET/MR)
GE (improved image recon, Motion Freeze)
PHILIPS (TOF, Wide Bore)
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Time-of-Flight Acquisition:
Principles
Δx =
Δt
c
2
C = 3*108 m/s
Back projection
along the LOR
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FUTURE
• Mixed PET prescriptions
• Partial volume correction
• Depth of interaction
• Amplitude gating
• Standardization of SUV
• New Radiopharmaceuticals
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Mixed acquisition protocol
Static
Gated
Static
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Effect of Object Size – Partial volume
Effect of object size on measured SUV
90
80
% True SUV
70
60
50
40
30
20
10
0
9
12
14
21
28
34
Lesion diameter (mm)
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Depth of interaction
From physics in Nuclear medicine Cherry, Sorenson and phelps
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Standardization of PET Imaging
decay-corrected dose/ml of tumor
SUV =
injected dose/patient weight in grams
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Measureable Parameters of Tumor
Growth and Malignancy
Gene expression
?
Receptors, kinases
DNA Synthesis/Hypoxia
RNA and Protein Metabolism
PET/CT 2007
Energy Metabolism
Vascularity
Blood Flow
Blood Brain Barrier
Cellular Transport
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Thank You
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